1. Field of the Invention
The present invention relates to a method and to an apparatus for manufacturing a fiber web, and, more particularly to dewatering of a fiber web wherein the fiber web is a web of tissue or hygiene material, provided with a three-dimensional surface structure.
2. Description of the Related Art
The imprinting of a three-dimensional structure into the surface of a paper web, in particular into the surface of a tissue web, more particularly into the surface of hand tissue, is known (see, for example, WO 99/47749, WO 01/18307). It is further known that a very good paper quality can be achieved by so-called through-air drying (TAD). However, it is disadvantageous that the use of TAD dryers is very complex and correspondingly expensive. What is needed in the art is a method of apparatus for dewatering of a fiber web of tissue or hygiene material, having a three-dimensional surface structure, which is less complex and less expensive.
To make the highest quality tissue and toweling products, it is necessary to develop products that are high in bulk, high in absorbency, yet still have adequate strength. The normal papermaking processes, which includes shoe and roll presses for dewatering a wet sheet, do not provide a bulky, absorbent sheet. Instead, they provide a strong, “flat” sheet that is typical of old technology, low cost tissue.
Several techniques are used to develop sheet bulk. Generally, the fiber web or sheet is first formed on, or vacuumed into, a special embossing or imprinting fabric. This fabric is rough, due to its coarse weave. The wet sheet conforms to this fabric and in doing so this increases the overall bulk of the sheet. Next, air is pulled through the sheet using a vacuum or low pressure. This airflow mechanically dewaters the sheet. Finally, hot air is blown through the sheet to dry it. The hot air dryer is called a Through Air Dryer (TAD for short). A TAD is usually made up of two large drums that by way of a vacuum pull heated air through the sheet, thereby drying it. These are very expensive units costing millions of dollars, to install.
As shown in earlier times, one way to get high bulk is to emboss or mold the sheet while it is wet. This can be done either by forming the sheet on a rough forming or molding fabric, or it can be formed “flat” in a conventional manor and then vacuumed into an embossing fabric. Either way, the sheet surface takes on the approximate shape of the embossing fabric surface. After the sheet is molded, it must be dried to its final state.
Drying is usually a two step process, where water is first removed mechanically, and then the remaining water is removed using heat. The problem is that it is difficult to mechanically remove water from the sheet without destroying its molded structure. If the sheet and fabric are pressed, for example, little water is removed since the embossing fabric adsorbs and then rewets the sheet after pressing. If the sheet is removed from the embossing fabric and then pressed, more water is removed, but the sheet bulk and absorbency is lost since the sheet becomes flatter.
The situation is slightly better if the sheet and embossing fabric are passed over a vacuum box. In this case, most prior art shows that the embossing fabric is on the vacuum side, supporting the sheet as air is pulled through it. The action of the vacuum removes water from the sheet, but after the water leaves the sheet, the embossing fabric retains much of it. Later, when the vacuum is removed, water passes back into the sheet, rewetting it. With this technology, the highest solids obtained for the sheet with low basis weights is less than 25% and more likely close to 20%.
Nevertheless, vacuum dewatering has been used since it retains the sheet structure. However because the sheet is so wet, this technology uses a lot of energy, in the form of a hot air, to dry the sheet.